Tall tale: Trees could power wireless networks

PORTLAND, Ore.  The U.S. Forest Service uses a network of automated stations to track weather conditions, especially during fire season. But the solar-powered remote network needs large forest clearings so the Sun can reach collectors.

At the same time, Forest Service officials wish they could install sensors on more trees.

Now, researchers say that by harnessing the voltage difference between a tree and the ground new ultralow-power sensors can transmit sensor data from almost any tree. The approach would also elimate the need to clear the forest floor for solar power.

"We believe that by installing wireless sensors on just four trees per acre, we can provide better fire prediction modeling, earlier alerts and much better local climate data than is available in any forest today," said professor Andreas Mershin of the Massachusetts Institute of Technology.

MIT researchers also predicted that tree power will eventually be useful for other types of environmental monitoring, such as watching the long expanse of forested border with Canada.

MIT scientists tried to determine why trees have a different voltage potential than the ground. After eliminating several other possibilities, such as electromagnetic radiation, they unearthed a principle from the 19th century: that pH differences can create a voltage potential.

"Its the imbalance in pH between the inside of the tree and the soil in which it was potted that generates a voltage," said Mershin. "You get about 59 millivolts for every step in pH mismatch."

What is significant about the voltage potential between a tree and its soil, according to Mershin, is that the metabolism of the tree itself works to maintain it; no matter whether its day or night, fall or spring, summer or winter, rain or sun.

"Because the tree has to have a certain concentration of these ions inside of it to be happy about its metabolic and other biological functions, the tree itself actively regulates its own pH. So, no matter what pH soil you put a tree in, it will work hard using its metabolic energy to keep its own pH constant," said Mershin.